In many fields the weakening of radiation--i.e., wave radiation,--in interaction with matter, provides the basis of measuring applications, where a part of the radiation is absorbed or weakened. The remainder is reflected or passed through the matter under observation. Depending on the characteristics and composition of the matter, radiation with a certain energy or wavelength, is completely absorbed or weakened. The absorbed energy is characteristic for the matter being analyzed. This characteristic absorption is typically evaluated to determine the characteristics of the radiated matter. For example, the elements in a compound may be determined with high accuracy. In addition, the concentration of a substance in a compound may be determined from the level of weakening of the characteristic radiation.
In electronic evaluation of transmitted or reflected signals weakened by characteristic absorption, the signal portions which are caused by disturbances of the measuring device, particularly the radiation source, and the signal portions caused by the absorption of the matter to be analyzed must be differentiated. In known procedures, particularly with low concentrations of the matter to be analyzed, the absorption signal is often superimposed by the noise of the measuring device. Improved resolution capacity can only be achieved with significant expenditures on instrumentation.
For example, EP 89 906 096.6 describes a method for determining the concentration of a gas in a gas mixture by applying a pulsated light of a laser, whose wavelength is switched after every pulse, and sent through the measuring path. One wavelength, or absorption wavelength, is characteristic for a gas whose concentration must be determined; the other wavelength is not characteristic for any substance which is located in the measuring path. The light pulse whose wavelength is characteristic for a selected gas is slightly weakened in the measuring path by the characteristic absorption.
With very low concentrations or short measuring paths, however, the useful signal becomes so small that it cannot be evaluated. It disappears in the noise of the overall system. In addition to the noise components generated from light detectors and associated electrical circuitry, there are significant noise components inherent to the laser radiation apparatus itself. For example, noise sources in the laser may be generated by the power supply, changes in pressure or temperature of the emission, and the influence of laser cooling, which may have been used on the emission. Depending on the laser type, this noise component may be minimized with significant expenditures in instrumentation, however, it can never be eliminated completely.
A problem in known procedures for determining the ammonia concentration in smoke gasses, is their sensitivity--the so-called cross-sensitivity--in relation to other smoke gas components. Particularly in waste incinerator installations, the smoke gas consists of many different substances. For example, many different hydrocarbon compounds from plastics incineration provide a wide spectrum of substances in the smoke gas.